Electric motors are frequently provided with brakes to stop the motor rapidly when the motor is de-energized and/or under emergency conditions. There are various types of arrangements for setting and releasing brakes. For example, a solenoid or the like can be energized to release the brakes when the motor is energized. The brakes are spring loaded to the set position so they set automatically when the motor and solenoid are de-energized.
Another way to operate the brakes is to have the brakes biased to the "set" position and manually actuated to the released position. The manual actuation can be a deadman control in which the operator's hand must keep the brake in the released position and the brakes will immediately set on release of the hand pressure.
Another type of manual control is one in which the control can be latched in a brake released position until the control is manually returned to the "set" position or will return automatically when a solenoid is energized.
No single construction permitted use in all the various modes. The designs were for specific uses and this required the motor manufacturer to carry a larger inventory and unit costs were necessarily high.